PROJECT SUMMARY Heart disease remains the leading cause of death in the United States and other developed countries. Half of these deaths occur suddenly, typically from ventricular tachyarrhythmias that arise in the setting of acute ischemia, acquired heart disease or inherited syndromes including channelopathies and cardiomyopathies. The specialized cardiac conduction system (CCS) comprises a heterogeneous network of cells that orchestrate the initiation and propagation of a wave of electrical excitation throughout the myocardium. Purkinje cells comprise the most distal component of the CCS and substantial, but indirect experimental data has accumulated supporting the concept that Purkinje cells play a key mechanistic role triggering a broad range of life-threatening ventricular arrhythmias. However, major gaps in our understanding of Purkinje cell biology have prevented this realization from being translated into novel anti-arrhythmic strategies. Through our recent identification of contactin-2, a novel cell adhesion molecule expressed in the conduction system, we have established new tools to identify, isolate and characterize murine Purkinje cells. Using these tools, we propose a series of studies to investigate contactin-2 dependent regulation of Purkinje network development, the regulation of cardiac electrophysiology by contactin-2 in normal and diseased hearts, and contactin-2 dependent mechanisms regulating pathologic remodeling in Purkinje cells.